Structured mesh generation and adjustment

Structured mesh generation and adjustment
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Visualization and preconditions

Click on the Show/hide mesh button in the tool bar.
If a mesh was already computed, it will be displayed. The button is gray, if mesh data are not yet available respectively were not saved after the last run. The availability of the unstructured mesh is the condition to start the structured mesh generation. The unstructured mesh should be generated before the structured mesh will be considered.
If the structured mesh was already created in any region, it will be indicated instead of the unstructured mesh if the structured mesh is used for computation of the selected actual variable. It is always the case for the Temperature and Convection .
The indication of the structured mesh only can be forced if one selects in the dialog Options for Grid the option " Show only structured Grid ".
Select Settings > Structured Grid.
The Unstructured Grid dialog opens:

Figure 12. The dialog window for generation and modification of the unstructured mesh

Mesh generation from scratch

If the structured mesh is already generated but you would like delete and begin the grid generation from scratch, it should be checked first, was there the phase tracking already done. If this was the case, then you should return the crystal and melt regions to the start configuration.

Figure 13. Hybrid mesh for Bridgmann configuration after the phase tracking procedure was done. The region boundary between the crystal and melt regions is deflected and coincides with the isotherm line crossing the triple point. The unstructured mesh is deformed in both crystal and melt regions. The structured mesh is deformed in the melt region. tracking results.

Press the button discard phase tracking result. You will be asked for confirmation because the computed shape of the phase boundary will be lost.

Figure 14. The dialog window for confirmation of the lost phase tracking results.

Press OK to continue. The nodes at the phase boundary will return to the start positions at the lines of the region boundary given by the CAD drawing before the phase tracking was started. Also the shifted nodes of the structured mesh and of the unstructured mesh in the crystal and melt regions will be returned to the initial positions.

Figure 15. Hybrid mesh for Bridgmann configuration after the results of the phase tracking procedure are discarded. The region boundary between the crystal and melt regions is returned to its started position which coincides with the CAD line of the drawing. The deformation of the unstructured and of the structured mesh is discarded.
Continue deleting of the structured mesh and press delete block structure button. After the structured mesh will be deleted all data stored on it will be lost. Therefore the dialog window will be opened and the confirmation will be asked.

Figure 16. The dialog window for confirmation of the lost phase tracking results.

The structured mesh will be deleted, only unstructured mesh should be visible now in the main CrysMAS window.

Figure 17. The Bridgmann configuration after the structured mesh was deleted in the melt region. The preserved first unstructured layer of the mesh is visible.

Note
The structured mesh cannot be deleted partially in the selected regions. The deletion and generation of the structured mesh is implemented only in the single run for all structured regions.
If no structured mesh is available, a zero number of blocks and control volumes on the structured mesh will be indicated in the left upper corner of the dialog window: "0 block(s) generated" and "0 Control Volumes". Select regions in the main CrysMAS window where you would like to generate the structured mesh. For selecting of the region or of the group of regions see Selecting Elements in Simulation Mode. The number of selected regions is indicated in the left upper corner of the dialog window.

Note
The selected regions should contain at most three different materials. For each material a separated computational domain with its own numerical parameters and user interface will be created. This allows the coupled computing of the fluid flow problems simultaneously in three different media, e.g. for the LEC configuration in semiconductor melt, gas and molten glass.

Note
The materials could have different name identifiers but treated as the same rheological media provided the name string of one material is fully contained in the name string of the other material. In this case both materials will be included into the same structured computational domain. This exception supports different materials with the same rheological properties while the other properties may differ. For example it is possible to introduce the transparent material "GAS-NAME" and the opaque one "GAS-NAME_opaque". Both materials will be included into the same domain. The opaque material can be prescribed to a narrow gap separating two gas filled cavities, that reduces the computational cost for the surface radiation. The user should take about the equality of the rheological properties in both materials.
After regions are selected and no special requirements to the mesh density and phase transition are needed, the structured mesh can be immediately generated. Deactivate the checkbox Grid refinement in the boundary layer" and press the button Generate block structure .

Figure 18. Main window: the structured block generated in the melt region of the Bridgmann configuration. It consists of the single numerical block. Structured Grid dialog window: no automatic mesh refinement and phase transition options were activated as it is shown.

The meshed melt region in the figure above has a rectangular shape, therefore a single block mesh could be generated. The number of numerical block and the total number of the control volumes on the structured mesh is indicated in the left upper corner of the dialog window as shown in the figure above.

Mesh adjustment

The mesh density can be changed everywhere on the structured mesh. The current distance between the nodes of the structured mesh is 0.005 m as indicated in the dialog field Default Grid: edge length (m) in the figure above. Enter another value into this field and press generate default grid .
The warning window appears. If some results were already computed on the structured mesh, the change of the default mesh density will affect the redistribution of the numerical fields because of the changed nodal numeration and the computed results will be corrupted. For confirmation press OK .

Figure 19. Warning window: changing of the default mesh density will affect the computed result on the structured mesh.

The structured mesh will be immediately regenerated and the mesh view in the main window will be refreshed. The changed number of control volumes on the structured mesh is indicated in the left upper corner of the dialog window.

Figure 20. Change of the default mesh density in the dialog window Structured Grid and the regenerated single block mesh in the main window.
The number of the edges can be changed locally at the boundary of the numerical block. The side of the block can consist of the single or few lines of the auxiliary CAD drawing. They can be selected by the middle button exactly as the lines of the main CAD drawing. The selected auxiliary line are marked by a red segment located parallel to the selected line with a small offset towards the inner of the block.
Press the middle mouse button in order to select a single auxiliary line. The number of selected lines is shown in the middle left part of the dialog window "1 line(s) selected" . The mesh properties at the auxiliary lines will be indicated in the grid properties at the line GUI box. The refinement factor and number of edges at the line are indicated.

Figure 21. Selection of the auxiliary line at the side of the numerical block.

The selected line at the symmetry axis of the melt in figure above is indicated by the red shifted line. It has 21 edges and the geometric progression factor is equal to 1. These parameters can be changed and the mesh generated again by pressing the button accept and regenerate grid.

Figure 22. Change of the structured mesh properties at the auxiliary line at the side of the numerical block.

Number of control volumes can be redistributed between two lines without any change of the numerical block size along its any side. This operation is available for any pair of lines of the auxiliary drawing which belong to the same side of the considered block. Additionally selected lines cannot be shared by any other neighbor block. It means that both lines should be located at the external boundary of the structured domain. This measure supports the manual improvement of the mesh orthogonality within the block. For using of this feature first a pair of auxiliary lines should be selected in the drawing.

Redistribution of the mesh density along the block side. The lower auxiliary line at the right block side is selected and indicated by a red color.

Figure 23. Redistribution of the mesh density along the block side. The lower auxiliary line at the right block side is selected and indicated by a red color.

Figure 24. Redistribution of the mesh density along the block side. The upper line is selected at the same external block side.

Redistribution of the mesh density along the block side. Two lines at the same block side are selected, whereas the lower line indicated in previous figure was selected first and the upper line from the previous figure as second.

Figure 25. Redistribution of the mesh density along the block side. Two lines at the same block side are selected, whereas the lower line indicated in previous figure was selected first and the upper line from the previous figure as second.

Redistribution of the mesh density along the same block side. The button "> >" is pressed and two mesh elements are transferred from the upper to the lower line.

Figure 26. Redistribution of the mesh density along the same block side. The button "> >" is pressed and two mesh elements are transferred from the upper to the lower line.

The mesh elements at the selected lines can be transferred as illustrated in the figures above. The button "< <" press acts the movement of the mesh in opposite direction as affected by the "> >" button. The button press transfers the mesh elements from the last selected line to the line selected before. The number of the mesh elements to be transferred is defined by the parameter in the dialog field "number of edges to shift" .

Note

For selection of two lines use the middle mouse button and CTRL keyboard button.

Phase transition

The phase tracking procedure can be applied for determination of the shape of the phase boundary crystal-melt. This procedure is enabled only if at the beginning of the structured mesh generation on activates this option by checking the button Phase transition located in the "Phase boundary" frame of the structured mesh dialog. If this wasn't done and the phase tracking procedure should be used, then the existed structured mesh should be deleted and generated with this option again.

For the phase tracking two regions in contact should be identified and marked as a crystal and a melt regions. The shared region boundary between these regions is treated as a start configuration of the phase boundary.

The material in both crystal and melt regions should consist of the single phase, which is correspondingly solid and liquid.

The crystal region is meshed only with the unstructured mesh and the melt region is meshed additionally with the structured mesh.

Both crystal and melt domains may consist of several regions but the phase tracking procedure is applied only to the pair of regions which are defined by the generation of the structured mesh. The structured and unstructured mesh will be moved only in this pair of regions.

The definition of the phase boundary affects also the heat source at the phase transition interface what is associated with the latent heat. Its value is proportional to the crystal growth velocity defined in the Process Parameters dialog. This heat source (or sink) will be applied in each thermal simulation even if the phase tracking procedure is not processed.

Another effect of the phase boundary definition is the possibility of the automatic setting of boundary conditions at the interfaces of the crystal and of the crucible in the Czochralski configuration, see Czochralski tab in the Process Parameters dialog.

The melt region where the structured mesh should be generated is selected. In the figure above this is a region with marked red boundary which corresponds to the melt in the Bridgmann configuration. More regions, also outside of the melt domain may be selected for the structured mesh generation.

Figure 27. Generation of the structured mesh with a phase transition. The melt region is selected for structured meshing.

The melt region shares its boundary with the crystal region below in the figure above. The structured mesh will be generated which has the non-matched boundary with respect to the external regions meshed only unstructurally. The only exception is the phase transition crystal-melt interface where the matched boundary between the structured mesh in the melt and the unstructured mesh in the crystal is necessary. Therefore the density of the unstructured mesh at the phase boundary already existed before the structured mesh generation will be taken over for the structured mesh.
If the mesh density at this region boundary is not sufficient for e. g. the numerical resolution of the curvature of the phase boundary crystal-melt, then first the number of edges should be increased at this line by the unstructured mesh regeneration. Use the "respect edge length" option in the unstructured mesh dialog to prescribe the mesh density at this line.
Note that also many lines at the shared phase boundary are allowed. Then one have more means for the mesh density redistribution along the phase boundary.
If the prepared unstructured mesh has sufficient quality, one may proceed with the structured mesh generation for hybrid mesh of the phase transition.
Figure 28. Structured mesh with a phase transition.

After selection of the regions to be meshed structurally the phase transition can be defined. Check the button phase transition and enter the numbers of the crystal and melt regions into the crystal and melt dialog fields. The region numbers are indicated at the bottom of the main graphical window of CrysMAS if the mouse cursor is positioned inside of the region and no isolines, vector arrows or color plots are activated. Press the button generate block structure in order to generate the structured mesh. The mesh will be generated so that the lines of it coincide with the vertexes of triangles at the phase boundary.

Note
The parameter "edge length (m)" in the Default Grid frame should be selected so that the resulting mesh density will not exceed the mesh density at the phase boundary which is fixed by the edges distribution on it. If it will be case, the mesh generation will be interrupted with an error message. The the parameter "edge length (m)" should be decreased and the mesh generation repeated.

Note
The mesh in the melt region can be also adjusted manually after the structured mesh was generated the first time. But in the melt region care should be taken in order to do not change the total number of control volumes at the block side opposite to the phase boundary, because this value should be equal to the fixed number at the phase boundary. Also no mesh changes at the block side coinciding with the phase boundary may be done.
If the structured mesh with the phase transition is already generated and the phase tracking has already worked, then the locations of nodes of the structured and of the unstructured meshes will be changed. Also the coordinates of the lines of the drawing will be changed. Therefore in the case if the structured mesh should be deleted and generated from scratch first the geometry should be returned to its original position before phase tracking has worked.
In order to delete the structured mesh with the defined phase transition and already worked phase tracking procedure, press the button discard phase tracking result in the Phase boundary frame. The warning dialog will occur.
Figure 29. Warning dialog by return to the not distorted configuration before the phase tracking work.

After the confirmation the configuration for the phase tracking action will be restored. The compute field data therefore will be preserved. Now the structured mesh can be deleted by pressing the button "delete block structure".
After all changes have been made, close the mesh dialog by clicking Close button.

Unstructured Grid